Computer networks allow users to access information stored within, or associated with, the network. A typical computer network can include multiple client computer devices that communicate with each other through a system of data communication devices (e.g., switches and routers) and transmission media (e.g., electrical wiring, fiber-optic cable, and/or wireless connections). In general, a transmitting client device exchanges data with a receiving client device by packaging the data using a standard format or protocol to form one or more data structures (e.g., packets, frames, or cells). The transmitting client device transfers these data structures to the receiving client device through the above-described network of computerized devices and transmission media.
Conventional computer networks, such as local area networks (LANs), can be subject to unwanted intrusion or attacks. For example, computerized devices on a network can be subject to user-level network attacks, such as drive-by-downloads (e.g., computer virus or malware) that can occur without a user's knowledge, such as when the user visits a particular website. Network computer devices can also be subject to other types of user-level network attacks, such as phishing attacks, which can obtain relatively sensitive information, such as passwords and user names, from the network computer devices.